Motor vehicles today are increasingly equipped with radar systems, in order to detect the distance of objects from the motor vehicle and the relative speed of said objects in reference to the motor vehicle. The detected distances and relative speeds may be used in various driver assistance systems. Such driver assistance systems may include automatic headlight range controls, the automatic selection of the light distribution pattern, an automatic adjustment of vertical or horizontal light-dark boundaries, brake assist systems, or the like, by way of example. In these radar systems, the device mentioned at the outset can be used with a voltage-controlled oscillator.
The European patent publication with the publication number EP 1 325 350 B1 discloses a method and a device for determining the distance and relative speed of an object distanced from the motor vehicle. The patent publication suggests a FMFSK method (frequency modulated frequency shift keying). This device, which uses the method, has a voltage-controlled oscillator which is used to generate multiple signals for transmission. Each signal has several segments with frequencies which differ from each other but are constant within the segment. The segments are transmitted consecutively, and the frequency of the signal segments of a signal are increased from one segment to the next. However, the segments of a signal are not transmitted directly one after the other. Rather, each segment of one signal is followed by a segment of another signal. In this case, the alternation between the signal segments always occurs in the same sequence, and is repeated in each clock cycle. The result is interleaved signal segments. The frequency is always kept constant during a signal segment.
In order to obtain the best possible results using this method, it is only possible to use voltage-controlled oscillators which have specific basic characteristics with respect to the rigidity, warping, aging, and temperature drift of the oscillator. For this reason, during the production of a device like the one described in this patent publication, all voltage-controlled oscillators are tested prior to assembly, and only suitable, controlled oscillators with the same characteristics are selected. In addition, during operation, cycles are inserted into the ongoing operation process at regular intervals, wherein the voltage-controlled oscillators are calibrated during said cycles in order to compensate for such things as temperature drift, for example. The calibration is carried out by means of a phase lock loop (PLL). The phase lock loop included for the purpose of calibration includes a means, also known as a phase detector, for finding a frequency difference, a means, also known as a filter, for the purpose of adjusting the voltage used to control the oscillator, the voltage-controlled oscillator, and optionally a frequency divider. During the calibration cycles the devices cannot be used for the purpose of generating radar signals to measure distance and relative speed. This is disadvantageous for the driver assist systems in that the driver assist systems cannot be provided with real-time data from the road during said period.